In general, a nitrile rubber is a random copolymer of acrylonitrile and butadiene, which is produced at an acrylonitrile content ranging from 15 to 50%.
The most essential advantage of nitrile rubber is considerably excellent oil resistance thereof. This is due to polarity of nitrile present in acrylonitrile. For this reason, physical properties of nitrile rubbers greatly vary depending upon amounts of nitrile incorporated in nitrile rubbers. Regarding correlations between oil resistance, nitrile content and swelling, and between aniline point (defined as the minimum temperature at which equal volumes of hydrocarbon and aniline are completely miscible wherein the aniline point enables estimation of components of hard mineral oils such as gasoline) of oils and swelling, as content of nitrile increases, oil resistance becomes better, and as aniline point increases, swelling decreases. As used herein, the term “oils” refers to mineral oils, that is, petroleum hydrocarbons which are widely used as lubricants or hydraulic operating oils. In particular, it should be noted that these oils are preferably used after evaluation of oil resistance of rubbers at an aniline point and measurement of variation in physical properties or swelling in oils used as seals, because the oils contain a variety of additives which may have any effect on the rubbers. In addition, as a result of measurement of compression set distortion of O-rings in hot oils for a long time, it can be seen that the O-rings exhibit superior properties and that O-rings slightly increasing in volume are presumed to exhibit superior sealing properties as compared to O-rings decreasing volume, based on the fact that O-rings slightly increasing in volume exhibit good properties. Nitrile rubbers are the most suitable for use in O-rings for seals of lubricating oils, operating oils, fuel oils or the like, V packings and oil seals in all fields including industrial machines, construction equipment, automation systems, cars, airplanes and the like and are used in great amounts due to excellent oil resistance. Temperature at which nitrile rubbers are used greatly varies according to mixing conditions, but low contents of nitrile are used for apparatuses, airplanes and the like operating at −50 to −120° C., in particular, at low temperatures in polar regions or cold climes. In addition, high contents of nitrile exhibit excellent heat resistance or mechanical properties and superior gas barrier properties, thus being applicable to vacuum conditions of 10 torr or less. Nitrile rubber is polymerized by common low-temperature emulsion polymerization and coagulated using a coagulant which is then used in the form of a veil. However, there is a need to solve a problem of low production efficiency due to repeated removal of residual foreign substances present on molds after molding of nitrile rubbers.